The steam cracking processes for the production of ethylene are common petrochemical processes. These processes are practiced throughout the world with millions of tons of ethylene product being produced annually. The ethylene steam cracking process typically involves thermally cracking a feedstock (such as ethane, propane, naphtha, gas oils, or even higher boiling point feedstocks) in a furnace at very high temperatures normally in the range of about 1382 to about 1742° F. (750 to 950° C.) in the presence of steam followed by rapidly cooling the thermally cracked stream to produce a product stream comprising ethylene.
Although it is desired to maximize the amount of ethylene produced in the process, many other compounds are produced as a byproduct of the cracking process. These byproducts generally range from elemental hydrogen, H2, as well as light hydrocarbon components, such as C1-C4 hydrocarbons to heavier hydrocarbon compounds including naphtha range materials and heavy oils including steam cracker tar.
The steam cracker tar is a high molecular weight, highly aromatic product that results from the production of ethylene. The steam cracker tar is the heavy fraction or “bottoms” fraction of the product produced by a steam cracking process typically with an initial boiling point of about 550° F. (288° C.). The amount of steam cracker tar that is produced is dependent upon the actual configuration of the ethylene steam cracker unit, but is also dependent upon the initial feed content utilized in the process. In general, the heavier the feedstock to the ethylene steam cracking process, the more heavy oils including steam cracker tar is produced. While it is preferable to use a light feedstock such as ethane, natural gas, or naphtha, the more limited availability and higher costs of these lighter ethylene cracker feedstocks have driven the ethylene cracking economics toward the use of heavier, higher molecular weight feed compositions to meet the growing demand for ethylene production. One significant drawback to utilizing a heavy feedstock such as a gas oil as the feedstream, is that the amount of steam cracker tar produced by the process can be significant and the product stream from the ethylene steam cracker furnace can contain more than about 15 wt % to more than 40 wt % steam cracker tar boiling range material.
This steam cracker tar product generally boils in the range from about 550 to about 1500° F. (288 to 816° C.) and has significant asphaltene content. Steam cracker tar is preferably sold as a fuel oil or bunker fuel but due to its high aromatic and asphaltene content the steam cracker tar stream can encounter compatibility problems with being blended with other fuel oils and bunker fuels and therefore often needs to be segregated and sold as a separate downgraded fuel product. This presents significant additional storage logistics and costs to a refiner over being able to blend the steam cracker tar into the standard fuel oil or bunker fuel product pool.
Therefore, there exists in the industry a need for an improved process for improving the properties of steam cracker tars for use as a blend stock for fuel oils or bunker fuels.